The present invention relates to a semiconductor module mounting structure, a cardlike semiconductor module, and heat receiving members bonded to the cardlike semiconductor module. The present invention is preferably applicable to a semiconductor module mounting structure for bonding a cardlike semiconductor module (i.e. a so-called double-sided semiconductor module having electrodes on both surfaces thereof) to the members to be bonded with thermal conductive grease.
To improve heat radiation properties of a power semiconductor module incorporating a semiconductor switching element chip used for electric power control, the Japanese patent application Laid-open No. 2000-243881 discloses a cardlike semiconductor module including two metallic radiator plates independently exposed on principal surfaces of the power semiconductor module. According to this conventional cardlike semiconductor module, two metallic radiator plates can serve as electrodes being independently connected to two principal electrodes of the above-described semiconductor switching element chip. In this respect, this kind of cardlike semiconductor module is referred to as a double-sided electrode type semiconductor module.
The double-sided electrode type semiconductor module is sandwiched between a pair of heat receiving members directly or via thin insulating sheets (resin films), so that a pair of metallic radiator plates of the power semiconductor module can be independently brought into contact with a pair of heat receiving members. With this arrangement, heat of the semiconductor switching element chip can be radiated or released to the heat receiving members via the metallic radiator plates (and the insulating sheets, if used).
According to this semiconductor module mounting structure, it is very important to reduce a contact thermal resistance between the metallic radiator plates of the power semiconductor module and the heat receiving members contacting with these metallic radiator plates. To this end, it is conventionally known to fill the gaps between the metallic radiator plates and the heat receiving members with appropriate thermal conductive grease having excellent thermal conductivity. For example, the Japanese patent application Laid-open No. 13-308263 (2001) discloses an inverter device having a semiconductor module bonding structure with applied grease.
According to the above-described conventional semiconductor module mounting structure, the semiconductor switching element chip is repetitively heated and cooled. The semiconductor module, especially the metallic radiator plates having large thermal expansion coefficients, will repeat expansion-and-contraction cycles, namely repetitively expand and contract in the surface direction as well as in the thickness direction.
When the semiconductor module causes repetitive expansions and contractions in the surface direction, no serious problems will arise. However, the repetitive expansions and contractions in the thickness direction will cause problems. More specifically, the repetitive expansions and contractions of the semiconductor module will forcibly and greatly change the size of small gaps defining grease layers (hereinafter, referred to as grease gaps) formed between the heat receiving members and the metallic radiator plates.
It is usual to set a pair of heat receiving members sandwiching the power semiconductor module so as to maintain a substantially constant distance therebetween. However, strictly speaking, the heat receiving members themselves cause thermal expansions and contractions. The distance between the heat receiving members decreases in high-temperature conditions. On the other hand, the distance increases in low-temperature conditions. As a result, in a condition that the metallic radiator plates of the semiconductor module and the heat receiving members have high temperatures, the grease gaps shrink and accordingly the grease is forced to shift along the heat dissipating surfaces of the metallic radiator plates and pushed out of the grease gaps between the heat receiving members and the metallic radiator plates. On the other hand, in a condition that the metallic radiator plates of the semiconductor module and the heat receiving members have low temperatures, the grease gaps expand and accordingly the grease is sucked into the grease gaps between the heat receiving members and the metallic radiator plates.
Such repetitive shifting of the grease in the surface direction will induce adverse invasion of external air into respective grease gaps and will leave undesirable air layers between the heat receiving members and the metallic radiator plates. The air layers will greatly increase the thermal resistance in the process of radiating or releasing the heat from the metallic radiator plates to the heat receiving members.
To solve this problem, it is desirable to arrange at least one of two heat receiving members so as to freely shift in the thickness direction in accordance with expansions and contractions of the cardlike semiconductor module occurring in the thickness direction. However, according to ordinary cardlike semiconductor module stack structures, flexibly changing the distance between the paired heat receiving members in accordance with expansions and contractions of the cardlike semiconductor module is very difficult because these heat receiving members are fixed in a casing or housing in many cases.
Alternatively, it may be possible to use bolts or other fasteners to tightly hold the cardlike semiconductor module between a pair of heat receiving members disposed in the thickness direction. This may bring the effects of reducing the elastic compressions of the cardlike semiconductor module and the heat receiving members so as to increase the thicknesses even when the cardlike semiconductor module and the heat receiving members shrink in their cooled conditions, and as a result suppressing changes of grease gaps. However, in view of strength of the cardlike semiconductor module, it is difficult to give the power semiconductor module a sufficient capability of elastically compressing.